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Immediately succeeding the Eagle Ford clays comes the Austin-Dallas chalk, which is the most persistent, and at the same time the most representative division of the whole Cretaceous system. The general character of this division has recently been described to the writer as follows:

"The rock of this formation is a massive, nearly pure, white chalk, usually free from grit, and easily carved with a pocket knife. Under the microscope it exhibits a few calcite crystals, and particles of amorphous calcite, and innumerable foraminiferæ. The air-dried indurated surfaces are white, but the subterranean mass has a bluish white color. The rock weathers in large conchoidal flakes, with an earthy fracture.

"In composition it varies from 85 to 94 per cent of calcium carbonate, the residue consisting of magnesia, silica, and a small percentage of ferric oxide, as can be seen from the following analyses of unselected specimens by the chemist of the Survey.

"The thickness of this chalk at Rocky Comfort is over 500 feet, 100 feet of which can be seen at the surface, the remaining 400 feet having been penetrated by bored wells. So far as observed in Texas it averages the same thickness at Austin, Sherman, and Dallas. It is of great uniformity throughout its massive thickness and extent, but it shows a few local differences in hardness, which are sometimes due to surface induration.

"It so closely resembles some of the beds of the underlying Comanche and of the overlying Upper Cretaceous that until recently they have not been differentiated. Upon close examination, however, it is noticeable that the Lower Cretaceous beds, as seen where Little River crosses the Choctaw boundary, are distinctly stratified and very much harder and generally more or less crystallized from pressure, solution, and redeposition of the carbonate of lime in the chalk. The topography of the Rocky Comfort beds is also of a milder type than that of the Comanche series, and is recognizable even at a distance. Above all, it is distinguished by its softness and by its entirely different fossil remains. The Rocky Comfort beds are also distingushed from the other chalky beds of the Upper Cretaceous by their greater firmness, different fossils, and by their higher percentage of calcium carbonate. With the exception of the White Cliff chalk, the other beds of the Upper Cretaceous seldom contain more than fifty per cent of calcium carbonate, the average being